Copper is one of the most widely used metallic materials of our time. Although copper is a semiprecious metal, this material is readily oxidizable, which often has an adverse effect on its use properties. This manifests itself not only visually but also has in particular practical technical disadvantages. Particular problems arise in the coating of printed circuit boards, which are then assembled in soldering processes, copper wires which are used as electrical conductors, or copper pipes. Finely divided copper powders are practically impossible to produce and use without oxidation protection.
Copper is normally not provided with protective coatings like iron and steel, which in the case of lacquers often have to be applied in several layers. Instead, as protection against copper corrosion, substances which form complexes with copper, such as for example imidazoles, benzimidazoles, benzotriazoles, thiourea and imidazole-2-thione, are predominantly used.
Such organic complexing agents are admittedly inexpensive and easy to process, however they display a number of disadvantages. Thus formulations with imidazoles or benzimidazoles often contain formic acid and sometimes other organic acids which smell unpleasant, are corrosive and have toxicological disadvantages. In addition, the thermal stability is low.
Therefore, in the production of printed circuit boards, for protection against corrosion copper is often coated with other metals such as for example gold, silver or tin, in order to preserve the solderability of the copper contacts and the copper-plated drill holes, which is lost in a very short time through oxidation.
An overview of common solderable final surfaces and their technical, economic, ecological and toxicological advantages and disadvantages are disclosed in “Alternative Technologies for Surface Finishing—Cleaner Technology for Printed Wired Board Manufacturers”, EPA, Office of Pollution Prevention and Toxics, June 2001, EPA 744-R-01-001.
Metallic coatings are in general very suitable for printed circuit boards, however they also display a number of disadvantages. Coatings with gold are expensive not only on account of the high gold price, but in addition require special processes for the application of the gold layer. For example, gold cannot chemically be applied in so-called horizontal plants but only in vertical plants, which results in additional high process costs.
The application of silver is poorly reproducible, and the necessary plants are difficult to regulate.
Tin is admittedly satisfactory from the technical and economic point of view in particular when it is applied with the aid of an organic metal, such as for example in the ORMECON CSN-process of Ormecon GmbH, however its deposition as a rule requires several minutes, which renders correspondingly large-sized plants necessary in order to ensure a high throughput.
From EP 0 807 190 B1, a process for the production of metallized materials is known, wherein the material to be metallized is first coated with an intrinsically conductive polymer, the intrinsically conductive polymer is then activated by reduction and finally the metal is applied in a non-electrochemical manner, in that the coated material is brought into contact with a solution of ions of the metal. The process is particularly suitable for the deposition of tin onto copper but also for the metallization of plastic surfaces.
EP 0 407 492 B1 discloses a process for the coating of substrates with thin layers of intrinsically conductive polymers, wherein for example polyaniline is non-electro-chemically deposited from a metastable dispersion onto a substrate. As substrates, inter alia metals such as gold, platinum, iron, copper and aluminium are mentioned. With metals that are less noble than silver, the layers of conductive polymer lead to the formation of metal oxide layers and should be suitable inter alia for corrosion protection.
EP 0 656 958 B1 concerns a process for the production of corrosion-protected metallic materials such as iron, steel, copper and aluminium, wherein a layer of an intrinsically conductive polymer is applied onto a metallic material and then the coated material is passivated with oxygen-containing water. It is pointed out that the application of the conductive polymer alone does not guarantee adequate corrosion protection, and the metallic material is therefore preferably provided with a corrosion-protecting coating after the passivation. The conductive polymer can be removed again before the application of the corrosion-protecting coating.